Composite structures are used in a wide variety of applications. In aircraft construction, composites are used in increasing quantities to form the fuselage, wings, tail section and other components. For example, the wings may be constructed of composite skin members to which stiffening elements such as stringers may be coupled to increase the bending strength and stiffness of the skin member. The stringers may extend in a generally spanwise direction along the wing. The stringers may be bonded to the skin members and may be configured to carry bending loads or loads that are oriented substantially perpendicularly relative to the skin member.
Stringers may be provided in a wide variety of cross-sectional shapes. For example, a stringer cross section may comprise a plurality of composite plies formed in a hat-section configuration having a base portion and a pair of webs extending outwardly from the base portion. The base portion may comprise a pair of flanges to facilitate coupling (e.g., bonding) of the stringer to the skin member such as the upper and lower wings skins of a wing. The hat-section stringer may include a cap which interconnects the webs and encloses the hat section in order to increase the torsional rigidity of the stringer. The cap also provides lateral stability to the webs against lateral bending or folding of the webs. At an intersection of each one of the flanges with one of the webs, a radius filler or noodle may be installed to enhance the load-carrying capabilities of the stringer.
The stringers in a wing may extend from an inboard section of the wing to an outboard section of the wing. Different loading conditions may be imposed on the wing at different locations along the wingspan. For example, at an inboard section of the wing, bending loads are typically higher than bending loads at an outboard section of the wing. In order to optimize the load carrying efficiency of the stringers and to minimize the occurrence of localized stresses in the skin members to which the stringers are coupled, it is typically desirable to reduce the stiffness of the stringer at the outboard section of the wing where the stringer may terminate. One method of reducing the stiffness of the stringer is to remove a portion of the cap. Removal of the cap from the stringer may also provide an opening in the stringer through which fuel vapors may be vented. In this regard, the stringer may provide secondary utility in addition to the primary load carrying function by acting as a conduit for venting fuel vapors from the inboard section of the wing near the fuel tanks to the outboard section of the wings.
However, for stringer cross sections where the web is oriented non-perpendicularly relative to the base portion, removal of the cap may necessitate a mechanism for maintaining the stability of the webs to prevent unwanted lateral bending. For example, the hat section of the stringer may comprise a cross-section having a trapezoidal configuration wherein each of the webs is angled inwardly toward one another and being interconnected by the cap. At locations where the cap is intact, the cap stabilizes the webs against such lateral bending or folding. However, at locations where the cap has been removed, the inwardly-angled webs are unsupported such that bending loads in the stringer may induce the webs to fold laterally inwardly.
Stabilizing the webs against lateral bending may also be necessary for stringers having a biased configuration in the ply layup. More specifically, when the stringer is viewed in cross section at the intersection of one of the webs with one of the flanges, the quantity of composite plies that make up the webs may be biased toward one side of the intersection or noodle. More specifically, when viewing a cross section of the composite plies that make up a thickness of the web, a larger quantity of plies may be positioned on one side of the intersection or noodle than on an opposite side of the noodle. The biased configuration may have undesired results.
Current techniques for stabilizing the webs include the use of metal (e.g., aluminum) fittings which may be mechanically fastened to the webs and flanges or skin members. Although generally satisfactory for their intended purpose, the use of such fittings presents certain drawbacks. For example, each one of the metal fittings must be individually fastened to the stringer using specialized mechanical fasteners which may require the formation of appropriately-sized holes in the fiber reinforced composite material which makes up the stringer and skin members. As opposed to conventional methods of forming holes in metallic structures, forming holes in composite materials and structures may also require the use of specialized tooling.
In addition, composites structures may require the installation of sleeved conductive fasteners. Such fasteners must typically be installed in a wet condition using a sealant to prevent galvanic corrosion between the dissimilar materials of the metallic fitting and the composite stringer/skin member. The wet installation of fasteners may further be required to prevent leakage across fasteners and/or to fill gaps between the fastener and the hole to allow for proper shear load transfer across the fastener and the hole. In addition, the use of metallic fittings may require the installation of sealant at the mating surfaces of the fitting and the stringer/skin member to prevent moisture buildup. Even further, in certain applications, fillet seals must be applied at the edges of the metallic fitting and the composite stringer to prevent moisture intrusion. As may be appreciated, the installation of metallic fittings in composite structures to stabilize the webs of a stringer may result in an increase in production time, increased part count, and an overall increase in the complexity of the structure.
As can be seen, there exists a need in the art for a system and method for stabilizing the webs of a stringer against lateral bending or folding which may otherwise occur as a result of a non-perpendicular orientation of the webs or due to a biased configuration in the composite plies that make up the stringer. Such stabilization may be required at locations where a cap of the stringer is not provided or which may occur at locations along the stringer where the cap has been removed. In this regard, there exists a need in the art for a system and method for stabilizing the webs of the stringer against lateral bending or folding which does not require the installation of separate metallic fittings.